The present invention relates to metal wires and the processes of obtaining them. These wires are used, for instance, to reinforce articles of plastic or rubber, in particular tubes, belts, plies and automobile tires.
The wires of this type which are presently/currently used are formed of steel containing at least 0.6% carbon, this steel having a strain-hardened perlitic structure. The rupture strength of these wires is about 2800 MPa (megapascals); their diameter varies generally from 0.15 to 0.35 mm, and their elongation upon rupture is between 0.4 and 2%. These wires are made by drawing an initial wire, known as a "machine wire", the diameter of which is about 5 to 6 mm, the structure of this machine wire being a hard structure formed of perlite and ferrite with a high content of perlite, generally more than 72%. Upon the production of this wire, the drawing is interrupted at least once in order to carry out one or more heat treatments which make it possible to regenerate the initial structure.
This process has the following drawbacks:
the raw material is expensive, since the carbon content is relatively high; PA0 the parameters cannot be easily modified; in particular, the diameter of the machine wire and the final diameter are maintained within strict limits, the process therefore lacking flexibility; PA0 the great hardness of the machine wire due to its strongly perlitic structure makes drawing prior to the heat treatment difficult, so that the rate of deformation .epsilon. of this drawing is necessarily less than 3; furthermore, the speed of this drawing is low and there may be breaks of the wire upon this operation. PA0 (a) it is formed at least in part by a steel having a carbon content of at least 0.1% and at most 0.6%, and a boron content of less than 8 ppm (parts per million); PA0 (b) the steel of the wire has a strain-hardened lower bainite type structure; PA0 (c) the diameter of the wire varies from 0.10 to 0.40 mm; PA0 (d) the resistance to rupture of the wire is at least 2800 MPa; PA0 (e) the elongation upon rupture of the wire is at least 0.4%. PA0 (a) a steel machine wire having a carbon content of at least 0.1% and at most 0.6% and a boron content of less than 8 ppm (parts per million) is strain-hardened, said steel comprising 28% to 90% proeutectoid ferrite and 72% to 10% perlite; the deformation ratio .epsilon. of this strain hardening being at least equal to 3; PA0 (b) the strain hardening is stopped and a single structural heat treatment is carried out on the strain-hardened wire; this treatment consists in heating the wire to above the AC3 transformation point in order to impart it a homogeneous austenite structure, then cooling it rapidly to a temperature of between 350.degree. C. and 450.degree. C., the rate of this cooling being at least equal to 250.degree. C./second, and maintaining it within this temperature range for a period of time of at least 30 seconds so as to obtain a structure of lower bainite type having carbide precipitates distributed practically uniformly in a ferrite matrix; PA0 (c) the wire is cooled to a temperature below 0.3 T.sub.F, T.sub.F being the melting point of the steel expressed in Kelvin; PA0 (d) a strain hardening is carried out on the wire which has undergone this heat treatment, the temperature of the wire upon the strain hardening being less than 0.3 T.sub.F, the deformation ratio e of this strain hardening being at least equal to 3.
Furthermore, the wires themselves sometimes have insufficient resistance to rupture and their resistance to fatigue is limited, due probably to a damaging of these wires upon the drawing before the heat treatment as a result of the great hardness of the machine wire.
The Japanese patent application published under No. 54-79119 describes a process of preparing a boron steel wire of bainite structure by heating in a fluidized bed. The wires obtained have poor mechanical properties.
The object of the present invention is to propose a strain-hardened metal wire of non-perlitic structure having a resistance to rupture and an elongation upon rupture at least as high as the known strain-hardened perlitic steel wires, and less damage than the known wires.
Another object of the invention is to propose a process not having the aforementioned drawbacks for the production of this wire.
The metal wire in accordance with the invention has the following characteristics:
The process of the invention for the production of this wire is characterized by the following features:
The invention also concerns assemblies comprising at least one wire in accordance with the invention.
The invention also concerns articles reinforced at least in part by wires or assemblies in accordance with the above definitions, such articles consisting, for instance, of tubes, belts, plies and automobile tires.
The invention will be easily understood on basis of the following embodiments and the diagrammatic figures relating to these examples.